Method for heating a crude carboxylic acid slurry in a post oxidation zone by the addition of steam

ABSTRACT

The present invention relates to a process for heating a secondary oxidizer with steam. More specifically, the present invention relates to a process for heating a secondary oxidizer with steam in a process for the production of a crystallized product.  
     A process to produce a crystallized product comprises the following steps (a) oxidizing in an primary oxidation zone an aromatic feedstock to produce a crude carboxylic acid slurry; (b) oxidizing said crude carboxylic acid slurry in a post oxidation zone in the presence of steam to form a post oxidation product; (c) purifying said post oxidation product to a decolorized post oxidation product; and (d) crystallizing in a crystallization zone said post oxidation product to form a crystallized product.

FIELD OF INVENTION

[0001] The present invention relates to a process for heating carboxylic acid slurry in a post oxidation zone by the addition of steam. More specifically, the present invention relates to a process for heating a terephthalic acid slurry in a post oxidation zone by the addition of steam in a process for the production of a crystallized product.

BACKGROUND OF THE INVENTION

[0002] Terephthalic acid (TPA) is commercially produced by oxidation of paraxylene in the presence of a catalyst, such as, for example, Co, Mn, Br and a solvent. Terephthalic acid used in the production of polyester fibers, films, and resins must be further treated to remove impurities present due to the initial oxidation of paraxylene. Typical commercial processes remove impurities by isolating a crude terephthalic acid solid, dissolving the solid in water at high temperatures and pressures, hydrogenating the resultant solution, cooling and crystallizing the terephthalic acid product out of solution, and separating the solid terephthalic product from the liquid as discussed in U. S. Pat. No. 3,584,039, herein incorporated by reference. Colored impurities from the benzil, anthraquinone, and fluorenone families are hydrogenated to colorless products and leave the process with the terephthalic acid solid product and wastewater streams.

[0003] Still other methods of obtaining a terephthalic acid product suitable as starting material for the production of polyesters do not involve a hydrogenation step. A method of producing a terephthalic acid product suitable for polyester production is to completely or nearly completely react paraxylene in a multiple stage oxidation process. In this invention, a unique and novel process is provided wherein a crude terephthaic slurry produced by the oxidation of an aromatic feedstock, typically p-xylene, is further oxidized in a multiple step process wherein heat is provided to a post oxidation zone by steam addition. The addition of steam to an oxidation zone subsequent to the primary oxidation zone results in a higher quality terephthalic acid product compared to conventional techniques.

SUMMARY OF THE INVENTION

[0004] This invention provides a process to produce a crystallized product from a crude carboxylic acid slurry by injecting steam in a post oxidation zone subsequent to the primary oxidation zone.

[0005] In a first embodiment of this invention, a process to produce a post oxidation product is provided comprising

[0006] (a) oxidizing in a post oxidation zone a crude carboxylic acid slurry in the presence of steam to form the post oxidation product; wherein the post oxidation zone comprises at least one post oxidation device; wherein thecrude carboxylic acid slurry comprises at least one carboxylic acid;

[0007] (b) purifying the post oxidation product to form a decolorized post oxidation product having b* color of less than 4.5.

[0008] In another embodiment of this invention, a process to produce a crystallized product is provided. The process comprising the following steps:

[0009] (a) oxidizing in a post oxidation zone thecrude carboxylic acid slurry in a post oxidation zone in the presence of steam to form a post oxidation product and an offgas stream; wherein the post oxidation zone comprises at least one post oxidation device;

[0010] (b) purifying the post oxidation product before or after the crystallization zone to form a decolorized post oxidation product having b* color of less than 4.5; and

[0011] (c) crystallizing in a crystallization zone the post oxidation product; wherein the crystallization zone comprises at least one crystallizer.

[0012] In another embodiment of this invention, a process to produce a crystallized product is provided. The process comprises oxidizing in a post oxidation zone a crude carboxylic acid slurry in the presence of steam to form the post oxidation product; wherein the crude carboxylic acid slurry comprises terephthalic acid; wherein the post oxidation zone comprises at least one post oxidation device; wherein the post oxidation product has a b* color of less than about 4.5.

[0013] In another embodiment of this invention, a process to produce a crystallized product is provided. The process comprises the following steps:

[0014] (a) oxidizing the crude carboxylic acid slurry in a post oxidation zone in the presence of steam to form a post oxidation product; wherein the post oxidation zone comprises at least one post oxidation device; wherein the pressure of the post oxidation zone is between about 10 barg to about 50 barg; and

[0015] (b) crystallizing in a crystallization zone the post oxidation product to form a crystallized product; wherein the crystallization zone comprises at least one crystallizer; wherein the crystallizer is operated at a temperature between about 140° C. and 190° C.; and wherein the crystallized product has a b* color of less than 4.5.

[0016] These objects, and other objects, will become more apparent to others with ordinary skill in the art after reading this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic of a process for heating a crude carboxylic acid slurry in a post oxidation zone by the addition of steam to produce a post oxidation product.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention provides a process to produce a post oxidation product comprising oxidizing a crude carboxylic acid slurry in the presence of steam to form the post oxidation product 120 and a offgas stream 80.

[0019] In a first embodiment of this invention, a process to produce a post oxidation product 120 is provided. The process comprises oxidizing in a post oxidation zone 90 a crude carboxylic acid slurry 30 in the presence of steam 100 to form the post oxidation product 120 and an offgas stream 80; wherein the post oxidation zone 90- comprises at least one post oxidation device; wherein the carboxylic acid slurry comprises at least one carboxylic acid; and wherein the post oxidation product 120 has a b* color of less than 4.5.

[0020] This oxidizing step and the post oxidation zone are described subsequently in this disclosure.

[0021] In another embodiment of the invention, a process to produce a crystallized product 180 is provided as shown in FIG. 1. The process comprises the following steps.

[0022] Step (a) comprises oxidizing the crude carboxylic acid slurry 30 in a post oxidation zone 90 in the presence of steam 100 to form a post oxidation product 120. Generally, the crude carboxylic acid slurry 30 is produced by oxidizing in a primary oxidation zone 20 an aromatic feed stock 10. The primary oxidation zone 20 comprises at least one oxidation reactor, and the crude carboxylic acid slurry 30 comprises at least one carboxylic acid.

[0023] The oxidizing in the primary oxidation zone 20 is completed under reaction conditions, which produces a crude carboxylic acid slurry 30 from an aromatic feedstock 10. Typically, the crude carboxylic acid slurry 30 comprises at least one carboxylic acid. Generally, the carboxylic acid is terephthalic acid.

[0024] Therefore, when terephthalic acid is utilized, the crude carboxylic acid slurry 30 would be referred to as crude terephthalic acid slurry. However, suitable carboxylic acids include, but are not limited to, terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, and mixtures thereof. Crude terephthalic acid slurry is conventionally synthesized via the liquid phase oxidation of paraxylene in the presence of metal oxidation catalyst. Suitable catalysts include, but are not limited to, cobalt, manganese and bromide compounds, which are soluble in the selected solvent. Suitable solvents include, but are not limited to, aliphatic mono-carboxylic acids, preferably containing 2 to 6 carbon atoms, or benzoic acid and mixtures thereof and mixtures of these compounds with water. Preferably, the solvent is acetic acid mixed with water in a ratio of about 5:1 to about 25:1, preferably between about 10:1 and about 15:1. However, it should be appreciated that other suitable solvents, such as those disclosed herein, may also be utilized. Patents disclosing the production of terephthalic acid, such as U.S. Pat. Nos. 4,158,738 and 3,996,271, are hereby incorporated by reference.

[0025] In cases where the carboxylic acid is terephthalic acid, the primary oxidation zone 20 has a water concentration less than about 10 wt %, preferably the water concentration is less than about 8 wt %, and most preferably the water concentration is less than about 6 wt %.

[0026] The post oxidation zone 90 comprises at least one post oxidation device. The crude carboxylic acid slurry 30 is fed to the post oxidation zone 90. The post oxidation zone 90 comprises at least one post oxidation device. The term post oxidation means that the oxidation occurs after the primary oxidation zone 20 discussed previously. For example, the post oxidation zone 90 can comprise post oxidation devices in series.

[0027] When the carboxylic acid is terephthalic acid, the crude carboxylic acid slurry 30 in the post oxidation device is heated with steam 100 to between about 180° C. and about 280° C., preferably between about 190° C. and about 240° C., and most preferably between 195° C. to 215° C. and further oxidized with air or a source of molecular oxygen fed by line 115 to produce a post oxidation product 120. The temperature is the internal temperature of the post oxidation device. When the post oxidation zone comprises more than one post oxidation device, the temperature can vary within the specified range for each post oxidation device. The steam 100 can be fed in the post oxidation zone 90 by any means known in the art. For example, a connection can be made for the steam 100 to be directly fed to the acetic acid vapor line. So the steam could be injected into the digester through the sparge ring. In addition to using steam, the post oxidation zone 90 can be heated with acetic acid vapor.

[0028] Additional air or molecular oxygen may be fed to the post oxidation zone 90 in an amount necessary to oxidize a substantial portion of the partially oxidized products and 4-carboxybenzaldehyde (4-CBA) in the crude carboxylic acid slurry 30 to the corresponding carboxylic acid. Generally, at least 70% by weight of the 4-CBA is converted to terephthalic acid in the post oxidation zone 90. Preferably at least 80% by weight of the 4-CBA is converted to terephthalic acid in the post oxidation zone 90. 4-carboxybenzaldehyde and p-toluic acid in high enough concentrations in the terephthalic acid product can be particularly detrimental to polymerization processes as they act as a chain terminator during the condensation reaction between terephthalic acid and ethylene glycol in the production of PET and can be detrimental to the performance of terephthalic acid hydrogenation processes. Typical terephthalic acid product contains on a weight basis less than 500 parts per million (ppm) 4-carboxybenzaldehyde and less than 250 ppm p-toluic acid. Preferably, the post oxidation zone is operated at a temperature and pressure sufficient that the b* color of the post oxidation product 120 ranges from about 0.5 to about 4.5.

[0029] Step (b) comprises purifying the post oxidation product before or after a crystallization zone to form a decolorized post oxidation product having b* color of less than 4.5.

[0030] The purifying of the post oxidation product can be accomplished by any means known in the art. For example colored impurities from the benzil, anthroquinone, and fluorenone families can be hydrogenated to colorless products. In addition any amount of process steps can be between the post oxidation zone and the purifying of the post oxidation product and the isolation or recovery of the post oxidation product if desired.

[0031] The b* color of the post oxidation product in conduit 180 is between about 0.5 to about 4.5. Preferably, the b* color of the decolorized post oxidation product in conduit 120 is between 0.5 to 2.0. Most preferably, the b* color in the decolorized post oxidation product in conduit 120 is between 0.5 to 1.5. The b* color is one of the three-color attributes measured on a spectroscopic reflectance-based instrument. The color can be measured by any device known in the art. A Hunter Ultrascan XE instrument is typically the measuring device. Positive readings signify the degree of yellow (or absorbance of blue), while negative readings signify the degree of blue (or absorbance of yellow).

[0032] Offgas from the post oxidation zone is withdrawn via line 80 and fed to a recovery system where the solvent is removed from the offgas. The solvent comprising volatile organic compounds (VOCs). The VOCs and any hazardous pollutants may be incinerated.

[0033] Step (c) comprises crystallizing the post oxidation product 120 in the crystallization zone 160 to form a crystallized product 180; wherein the crystallized product 180 has a b* color of less than 4.5.

[0034] Generally, the crystallization zone 160 comprises at least one crystallizer. Vapor product from the crystallization zone 160 is withdrawn via line 130, condensed in a condenser zone 170, which comprises at least one condenser and returned to the crystallization zone 160. Optionally, the liquid in conduit 140 or vapor 130 in the condenser zone 170 can be recycled, or it can be withdrawn or sent to an energy recovery device. In addition, the liquid crystallization offgas 150 from the condenser zone 170 is removed via line 150 and can be routed to a recovery system where the solvent is removed and crystallization offgas 150 comprising VOCs and pollutants are burned.

[0035] When the carboxylic acid is terephthalic acid, the post oxidation product 120 from the post oxidation zone 90 is withdrawn via line 120 and fed to a crystallization zone 160 comprising at least one crystallizer where it is cooled to a temperature between about 110° C. and about 190° C. to form a crystallized product 180, preferably to a temperature between about 140° C. to about 180° C., most preferably 150° C. to 170° C. Preferably, the b* color of the crystallized product in conduit 180 is between about 0.5 to about 4.5. More preferably, the b* color of the crystallized product in conduit 135 is between 0.5 to 2.0. Most preferably, the b* color in the crystallized product in conduit 135 is between 0.5 to 1.5

[0036] The crystallized product 180 from the crystallization zone 160 is withdrawn via line 180. Typically, the crystallized product 180 is then fed directly to a vessel and cooled to form a cooled crystallized product. When the carboxylic acid is terephthalic acid, the cooled crystallized product is cooled in a vessel to typically a temperature of approximately 90° C. or less than before being introduced into a process for recovering the carboxylic acid as a dry powder or wet cake.

EXAMPLES

[0037] The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Example 1

[0038] In a plant trial, steam was used in the heating of a post oxidation device. The temperature of the post oxidation device was operating at about 195° C. to about 215° C. The airflow was set at a ratio of about 3.8 kg air/ton crude carboxylic acid slurry. The steam flow to the post oxidation device was between about 6.5 to about 8.2 metric tons/hour. An overall reduction in energy of 0.3 GJ/ton for producing terephthalic acid was observed. This represents a 5% reduction in energy versus operating without the use of steam to the post oxidation device. The acid burn and decomposition in the post oxidation device decreased due to the injection of steam into the post oxidation device. The acid loss (acetic acid/ton post oxidation product) dropped about 10%. Therefore, both reduction in energy consumption and acid burn were observed through the use of steam to heat the post oxidation device. 

We claim:
 1. A process to produce a post oxidation product comprising the following steps: (a) oxidizing in a post oxidation zone a crude carboxylic acid slurry in the presence of steam to form said post oxidation product; wherein said post oxidation zone comprises at least one post oxidation device; wherein said crude carboxylic acid slurry comprises at least one carboxylic acid; (b) purifying said post oxidation product to form a decolorized post oxidation product having b* color of less than 4.5.
 2. A process according to claim 1 where said decolorized post oxidation product has a b* color less than 3.5.
 3. A process according to claim 1 wherein said carboxylic acid is selected from the group consisting of terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, and mixtures thereof.
 4. A process according to claim 1 wherein said carboxylic acid is terephthalic acid.
 5. A process according to claim 4 wherein said steam is used to heat said post oxidation zone to a temperature in a range from about 180° C. to about 280° C.
 6. A process according to claim 5 wherein said post oxidation zone is operated at a pressure of about 10 barg to about 50 barg.
 7. A process to produce a crystallized product said process comprising the following steps: (a) oxidizing in a post oxidation zone said crude carboxylic acid slurry in a post oxidation zone in the presence of steam to form a post oxidation product and an offgas stream; wherein said post oxidation zone comprises at least one post oxidation device; (b) purifying said post oxidation product before or after a crystallization zone to form a decolorized post oxidation product having b* color of less than 4.5; and (c) crystallizing in said crystallization zone said post oxidation product; wherein said crystallization zone comprises at least one crystallizer.
 8. A process according to claim 7 wherein said purified post oxidation product has a b* color of less than
 3. 9. A process according claim 7 wherein said carboxylic acid is selected from the group consisting of terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, and mixtures thereof.
 10. A process according to claim 7 wherein said carboxylic acid is terephthalic acid.
 11. A process according to claim 10 wherein said steam is used to heat the post oxidation zone to a temperature in a range from about 180° C. to about 280° C.
 12. A process according to claim 11 wherein said post oxidation zone is operated at a pressure of about 10 barg to about 50 barg.
 13. The process according to claim 10 wherein said crystallizer is operated at a temperature between about 140° C. and 190° C.
 14. A process to produce a post oxidation product comprising oxidizing in a post oxidation zone a crude carboxylic acid slurry in the presence of steam to form said post oxidation product; wherein said crude carboxylic acid slurry comprises terephthalic acid; wherein said post oxidation zone comprises at least one post oxidation device; wherein said post oxidation product has a b* color of less than about 4.5.
 15. A process according to claim 14 wherein said steam is used to heat the post oxidation zone to a temperature in a range from about 180° C. to about 280° C.
 16. A process according to claim 15 wherein said post oxidation zone is operated at a pressure of about 10 barg to about 50 barg.
 17. A process to produce crystallized product said process comprising the following steps: (a) oxidizing said crude carboxylic acid slurry in a post oxidation zone in the presence of steam to form a post oxidation product; wherein said post oxidation zone comprises at least one post oxidation device; wherein the pressure of said post oxidation zone is between about 10 barg to about 50 barg; and (b) crystallizing in a crystallization zone said post oxidation product to form a crystallized product; wherein said crystallization zone comprises at least one crystallizer; wherein said crystallizer is operated at a temperature between about 140° C. and 190° C.; and wherein said crystallized product has a b* color of less than 4.5.
 18. A process according to claim 17 where said crystallized product has a b* color of less than
 3. 